This invention relates to mercury vapor fluorescent lamps and particularly to a method for maintaining the mercury pressure within the lamp at an optimum value of monitoring and controlling the actinic output of the lamp.
In a mecury fluorescent lamp, an electrical discharge is generated in a mixture of mecury vapor at low pressure and a fill gas typically argon, neon, Krypton, xenon or mixtures thereof. The light output from the lamp depends, among other variables, on the mercury vapor pressure inside the lamp tube. The primary radiation from the mercury is at 2537 Angstroms and arises from the transition between the lowest non-metastable excited state and the ground state. This ultraviolet radiation at 2537 Angstroms excites a phosphor which is coated inside the tube walls. The excited phosphor thereupon emits radiation at some wavelength, in the visible spectrum, characteristic of the phosphor.
It is known in the prior art that the optimum mercury pressure for maximum light output of a fluorescent lamp approximately 7 mtorr (independent of current) which corresponds to a mercury cold spot temperature of approximately 40.degree. C. (.apprxeq.100.degree. F.). At this temperature and pressure, the light output increases monotonically with the current. At cold spot temperatures higher or lower than the optimum, light output falls off.
It is therefore desirable to maintain the mercury pressure at the optimum at any lamp current and at any ambient temperature. Prior art techniques for accomplishing this function required a temperature-sensitive device such as a thermocouple, thermistor or thermostat to monitor the temperature of the cold spot. A feedback circuit provided closed loop control of a temperature-regulating device to maintain the optimum mercury pressure. These methods, although providing a closed loop control of the cold spot temperature sensor, must rely on a consistent relationship of cold spot sensor temperature to light output which may not exist under all conditions.
The present invention is directed to a novel method for maintaining optimum mercury pressure which does not require the use of cold spot temperature measuring devices. As will be demonstrated in the succeeding descriptive portion of the specification, if lamp current is kept constant, as mentioned above, the light output of the lamp (e.g. the phosphor output and, in some cases, the actinic energy made up of the phosphor and some of the mercury line energy) is a function of the mercury cold spot temperature. The optimum cold spot temperature is that which results in a peak or maximum light output. According to one aspect of the invention, the light output is continually monitored by a detector which is adapted to feed back a signal to a cold spot, temperature-regulating device under certain conditions. A control system responds to any reduction in the light output by reversing the operating mode of the temperature-regulating device. Thus, if the device has been off it is turned on and if on, it is turned off. Either action has the effect of restoring the light output to its peak level, and hence restoring the optimum mercury pressure.
A prime advantage of the method of the invention is that the system does not require any absolute calibration; that is, the peak light output for a particular lamp does not need to be determined. The system can sense and maximize the light output and provide constant maximum exposure for any current level. Further, the feedback circuit is extremely fast relative to the prior art feedback loop which required a longer response time due to the thermal mass of the mercury pool heat sink, the glass envelope and the temperature sensitive device.
The present invention is therefore directed to a monitoring and control system for optimizing and controlling the light output of a fluorescent lamp containing an excess of mercury at a cold spot therein, said system comprising:
a power supply for applying operating current to said lamp, PA1 temperature control means adapted to operate in a first mode whereby temperature at said cold spot is increasing and in a second mode whereby temperature at said cold spot is decreasing, and PA1 a monitoring means for detecting a drop in the light output of said lamp, said monitoring means adapted to transmit a signal to said temperature control means changing the instant mode of operation.